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Ionizing Radiation Induced Catalysis on Metal Oxide Particles M. A. Henderson, S. A. Chambers, Y. Su, J. L. Daschbach, Y. Wang, C. H. F. Peden, C. L. Perkins,(a) Prof. M. E. Castro,(b,c) Prof. A. K. Datye,(b,d) Prof. J. Osterwalder(b,e) Supported by Environmental Management Science Program (EMSP), DOE. (a) Postdoctoral Research Fellow. (b) EMSL User. (c) University of Puerto Rico. (d) University of New Mexico. (e) University of Zurich. This project focuses on a novel approach for destroying organics found in high-level mixed waste prevalent at DOE sites. In this project, we examine the destruction of organics using reduction/oxidation (redox) chemistry resulting from electron-hole (e-/h+) pairs generated in stable, wide bandgap semiconductors via interactions with ionizing radiation. Conceptually, this process is an extension of visible and near-UV (ultraviolet) photocatalytic processes known to occur at the interfaces of narrow bandgap semiconductors in both solution and gas phases. In these processes, an electron is excited across the energy gap between the filled and empty states in the semiconductor. The excited electron does reductive chemistry and the hole (whence the electron was excited) does oxidative chemistry. The energy separation between the hole and the excited electron reflects the redox capability of the e-/h+ pair, and is dictated by the energy of the absorbed photon and the bandgap of the material. The use of ionizing radiation overcomes optical transparency limitations associated with visible and near-UV illumination (g-rays penetrate much farther into a solution than UV/Vis light), and permits the use of wider bandgap materials (such as ZrO2), which possess potentially greater redox capabilities than those with narrow bandgap materials. We have examined in detail the radiocatalytic destruction of EDTA, a typical chelating agent found in DOE tank waste, over various TiO2 catalysts, with similar studies conducted on smaller probe organic molecules (such as ethanol, formic acid, and acetic acid). Our results show that solution-phase radiolytic processes dominate those associated with the TiO2 surface to the extent that any catalytic effects are not detected. This contrasts with the catalytic destruction of EDTA, which was greater than solution-phase radiolysis. UV-based experiments did show significant destruction of these small organics, in agreement with the literature. The presence of platinized TiO2, however, significantly increased the production of hydrogen during the destruction of these small organics, presumably through the interaction of solution-phase radicals with the catalyst surface. We have also shown that radiocatalytic reductive deposition of noble metals, such as Pt, onto TiO2 can explain the generation of benzene from tetraphenylborate (TPB) in Savannah River waste tanks. Our studies show that radiation (either ionizing or not) is not required for TPB decomposition on noble metal impregnated TiO2; i.e., the thermal decomposition is highly facile even at room temperature. However, when noble metal ions, which are known to exist in the waste along with colloidal titanates, are irradiated, they reductively precipitate on the titanates, opening the door for thermal decomposition of TPB. To understand better on a molecular level how redox processes occur on TiO2 surfaces and how surface structure influences these processes, we have conducted a variety of ultrahigh vacuum (UHV) studies on model TiO2 surfaces, focusing on the role of defects in surface chemistry. Because molecular oxygen is key in photo-oxidation processes, we have examined the interaction of molecular oxygen with the TiO2(110) surface using temperature programmed desorption (TPD), isotopic labeling studies, sticking probability measurements, and electron energy loss spectroscopy (ELS). Molecular oxygen does not adsorb on the TiO2(110) surface unless surface oxygen vacancy sites are present. These vacancy defects are generated by annealing the crystal at 850 K. The O2 saturation coverage at 120 K, as estimated by TPD uptake measurements, is approximately three times the surface vacancy population. Coverage-dependent TPD shows little or no O2 desorption below the vacancy population, presumably due to dissociative filling of the vacancy sites in a 1:1 ratio. A first order O2 TPD peak appears at 410 K above the vacancy coverage. Oxygen molecules in this peak do not scramble oxygen atoms with either the surface or with other co-adsorbed oxygen molecules. ELS measurements suggest charge transfer from the surface to the O2 molecule based on disappearance of the vacancy loss feature at 0.8 eV and the appearance of a 2.8 eV loss that can be assigned to an adsorbed o2- species based on comparisons with TiO2 inorganic complexes in the literature. Using results from recent spin-polarized DFT calculations in the literature, we propose a model in which three O2 molecules are bound in the vicinity of each vacancy site at 120 K. Based on the variety of oxygen adsorption states observed in this study, further work is needed to determine which oxygen-related species play important roles in chemical and photochemical oxidation processes on TiO2 surfaces. We have also examined the interaction of these oxygen-related species with a simple organic species, methanol. Although some methanol molecules reversibly dissociate on TiO2(110), the majority of the adlayer is molecularly adsorbed. New channels of chemistry were observed when CH3OH was adsorbed on the surface after o2 adsorption at various temperatures. o2 exposure at 300 K resulted in o adatoms (via dissociation at vacancies) that led to increased levels of ch3o-h bond cleavage, with the higher coverages of methoxyl resulting in disproportionation to form CH3OH and h2co above 600 K. In contrast, low temperature o2 exposure results in oxidation of CH3OH to h2co by C-H bond cleavage. These results provide incentive to consider alternative thermal and photochemical oxidation mechanisms that involve the interaction of organics and oxygen at surface defect sites. Working with Jürg Osterwalder at the University of Zurich, we have performed experiments mapping the 3-D Fermi surface of rutile TiO2(110). Our recent investigations at PNNL suggest that the reduction of TiO2 surfaces may be an important step in the radiation-induced chemistry that occurs under aqueous conditions. The reduction of TiO2 results in the formation of a Fermi surface. The Fermi surface is an important quantity because it determines many of the properties of the material (e.g., the chemical properties, the electrical and thermal conductivities, etc.). Experiments were performed on reduced TiO2 to determine the dispersion of the defect state and perform Fermi-surface mapping, ultraviolet photoelectron diffraction, and x-ray photoelectron diffraction. Our preliminary results suggest that point defects such as oxygen vacancy sites, although localized entities from a structural perspective, have longer-ranging influence on the electronic structure of the TiO2 surface. This delocalization of electrons at point defects has significant influence on the adsorption and reaction of molecules, and may also influence the trapping of photoexcited charge carriers. We have measured the dispersion of the defect state along several different directions to experimentally investigate this effect. Results show that although the highest intensity along k// is at nearly -0.8 eV, a significant shift occurs from -0.9 to -0.8 eV at k// = 1.2 to 1.5. This small but significant dispersion in the defect state is the first experimental evidence for delocalization of this state for reduced TiO2. William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: April 17, 2000 Security & Privacy PNNL-13147